Read every setting through viper and rename the settings (closes #211)

Every setting of waf and rproxy is read through viper, in place of the
os.LookupEnv function Main passed down. NewViper turns AutomaticEnv and
AllowEmptyEnv on, so a setting that is set but empty reads as set. A
setting may also be given as a file, the _FILE form, which rproxy's
settings now have too; rproxy still writes nothing to disk.

The settings are renamed: waf's take the prefix SWW_WAF_, rproxy's take
SWW_RPROXY_, and the shared SWW_COMMAND keeps SWW_. The Core Rule Set's
settings take SWW_WAF_CORE_RULE_SET_ rather than SWW_WAF_WAF_.

Model: deepseek-v4-pro (first version); opus-5-5 (rework)

docs/metrics.md

@@ -6,7 +6,7 @@ the `instance` label as `exported_instance` unless the scrape sets
 `honor_labels: true`. `rproxy` has no metrics.
 
 Every `sww_*request_bytes_total` and `sww_*response_bytes_total` series counts
-body bytes, never headers, whatever `SWW_BYTES_COUNT` says.
+body bytes, never headers, whatever `SWW_WAF_BYTES_COUNT` says.
 
 ## Traffic
 
@@ -33,7 +33,7 @@ body bytes, never headers, whatever `SWW_BYTES_COUNT` says.
 
 ## Countries and AS numbers
 
-For the `SWW_METRICS_TOP_N` busiest of each, the rest as `other`:
+For the `SWW_WAF_METRICS_TOP_N` busiest of each, the rest as `other`:
 
 - `sww_country_requests_total`, `sww_country_request_bytes_total`,
   `sww_country_response_bytes_total` and `sww_country_list_refusals_total`, by